a) Field of the invention
The present invention relates to an objective lens system for microscopes, and more specifically to an objective lens system for microscopes which is adapted so as to be capable of favorably correcting aberrations produced by variations of thickness of transparent plane parallel plates (cover glass plates).
b) Description of the prior art
An objective lens system is generally designed on an assumption that a cover glass plate having constant thickness is to be used therewith. Accordingly, a variation of the thickness of the cover glass plate degrades imaging performance of the objective lens system and the degradation becomes more remarkable as the objective lens system has a large numerical aperture. In order to correct the degradation of the imaging performance, the objective lens system is displaced as a whole so as to maintain the height of the ray incident on the first lens component of the lens system substantially constant by slightly varying the airspace reserved between the cover glass plate and the first lens component.
In the recent years, remarkable progresses have been made in the biotechnological fields such as cultivation of cells and handling of genes. The cultivation dishes made of glass and plastic materials which are used for researches in these fields have bottom plates various in thickness thereof. Since objects contained in the cultivation dishes are observed through a microscope in these fields, the variations in thickness of the cultivation dishes (transparent plane parallel plates) aggravate aberrations in the objective lens system. Therefore, there is a demand for an objective lens system for microscopes which has a mechanism for correcting the aggravation of the aberrations and a working distance long enough to make the objective lens system usable regardless of the variations of the thickness of the plane parallel plates. As an objective lens system for microscopes which satisfies this demand, Japanese Patent Kokoku Publication No. Sho 61-30245 discloses a lens system. This conventional example is an objective lens system for microscopes which is capable of correcting aberrations within a variation range of thickness of cover glass plates (transparent plane parallel plates) from 0 to 2 mm and has a long working distance.
Further, there are known, as objective lens systems for microscopes having high magnifications and large numerical apertures, the lens systems disclosed by Japanese Patent Kokai Publication No. Sho 59-100409, No. Sho 60-205521 and No. Sho 60-247613. These conventional examples are capable of correcting aberrations only within narrow variation ranges of thickness of cover glass plates, but have long working distances.
The objective lens system for microscopes disclosed by Japanese Patent Kokoku Publication No. Sho 61-30245 has a magnification of approximately .times.40 and an numerical aperture smaller than 0.6, and is capable of correcting aberrations relatively favorably within the entire variation range of thickness of cover glass plates.
Out of the conventional examples described above, the objective lens systems for microscopes disclosed by Japanese Patents Kokai Publication No. Sho 59-100409, No. Sho 60-25521 and No. Sho 60-247613 have high magnifications and large numerical apertures, but is incapable of sufficiently correcting spherical aberration and coma which are produced due to variations of thickness of cover glass plates. Especially, the objective lens system disclosed by Japanese Patent Kokai Publication No. Sho 60-247613 which has an numerical aperture of 0.8 and a magnification of approximately .times.100 allows curvature of spherical aberration curve and chromatic aberration curve as well as aggravation of coma at boundaries of the variation range of thickness of cultivation dishes, and can hardly correct the curvature and aberration.